Acute and chronic diarrheal diseases are major contributors to mortality and morbidity in developing countries where they are the second most frequent cause of infant mortality and to morbidity in the USA where they represent a huge economic burden. Lacking approved, safe and highly effective anti-diarrheal drugs, a NIH Single Topic Conference held 9/2011 concluded that development of drug therapy to treat all forms of diarrhea has a high research priority with the goal of reducing the duration and amount of diarrhea. The needed information that was lacking for diarrheal drug development was not felt to be in our understanding of the pathophysiology of diarrhea as due to inhibition of Na+ absorption with or without stimulation of Cl- secretion. Rather it was lack of information about the distribution and extent of expression of major ion transport proteins in the normal human intestine, how this changes with age, and how transporter activity and expression changes with various diarrheas. This R24 represents a collaborative effort of 5 principle investigators from three institutions all of whom have a primary research interest in intestinal epithelial transport and its regulation, but who bring expertise in several different but complementary areas of research which are needed to address the aims of this project. In this way we propose to provide the scientific underpinning needed for current and future development of anti-diarrheal drugs. The Aims of this project are to I. Develop a catalogue of the intestinal expression of potential anti-diarrheal drug targets in healthy controls and diarrheas using archived human intestine, mouse intestine and human enteroids; standardize model diarrheas in mouse small intestine and human enteroids; and perform a phenotypic screen using human enteroids to identify unique drugs to treat the diarrhea models. II. Determine changes in function of specific Na+ absorptive and Cl- secretory transporters in the diarrhea models and the mechanisms by which they occur, including effects of the newly identified anti-diarrheal drugs. III. Determine the efficacy of novel anti-secretory and pro-absorptive therapeutics in in vivo and ex vivo diarrheal models. High throughput screen for NHE3 stimulators.